Various types of biometric systems are used more and more in order to provide for increased security and/or enhanced user convenience.
In particular, fingerprint sensing systems have been adopted in, for example, consumer electronic devices, thanks to their small form factor, high performance and user acceptance.
Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important issues.
All capacitive fingerprint sensors provide a measure indicative of the capacitance between each of several sensing structures and a finger placed on or moved across the surface of the fingerprint sensor.
Some capacitive fingerprint sensors passively read out the capacitance between the sensing structures and the finger. This, however, requires a relatively large capacitance. Therefore such passive capacitive sensors are typically provided with a very thin protective layer covering the sensing structures, which makes such sensors rather sensitive to scratching and/or ESD (electro-static discharge).
U.S. Pat. No. 7,864,992 discloses an active capacitive fingerprint sensing device in which a driving signal is injected into the finger by pulsing a conductive drive structure arranged in the vicinity of the sensor array and measuring the resulting change of the charge carried by the sensing structures in the sensor array.
Although the fingerprint sensing system according to U.S. Pat. No. 7,864,992 provides for an excellent combination of fingerprint image quality and sensor protection, it would, in some applications be desirable to be able to acquire a high-quality fingerprint image without the use of a separate conductive drive structure. In particular, there appears to be room for improvement for “difficult” fingers, such as dry fingers.
An alternative active fingerprint sensing device is described in the paper “A 500 dpi Capacitive-Type CMOS Fingerprint Sensor with Pixel-Level Adaptive Image Enhancement Scheme” by Kwang-Hyun Lee and Euisik Yoon (ISSCC 2002/session 21/TD: Sensors and microsystems/21.3). In this fingerprint sensing device, an excitation pulse is applied to the sensing electrode of each pixel instead of to the finger. The potential of the finger is assumed to be substantially constant.
This fingerprint sensor would appear to be usable without a separate conductive drive structure. However, the fingerprint sensor described in “A 500 dpi Capacitive-Type CMOS Fingerprint Sensor with Pixel-Level Adaptive Image Enhancement Scheme” is said to be configured to exhibit a capacitance to the finger (the capacitance that is measured) in the range of 0 fF to 200 fF. In the field of fingerprint sensing, this is a relatively large capacitance, which indicates that the protective coating provided on top of the sensing electrodes is very thin. Actually, the protective coating is referred to as a “passivation layer”, which is generally understood to be a layer of SiO or SiN that has a thickness of around 1 μm. A fingerprint sensor with such a thin protective coating would not be robust enough for many important applications, including mobile device applications.
It would thus be desirable to provide a more robust capacitive fingerprint sensing device in which the excitation pulse is applied to the sensing electrode, which is at the same time capable of achieving a high quality fingerprint representation.